Effects of symmetry of urban street canyons on their inner airflow fields

Based on Reynolds stress turbulence model,airflow fields within urban street canyons under different building heights were simulated.The numerical simulation results show that the symmetry of the street canyons has significant effects on inner airflow fields.There is a counter-clockwise vortex in the lower part of higher frontage and lower back canyon,and there is a clockwise vortex in the upper part.The deeper the canyon is,and the better the airflow field develops.There is an obvious stagnation point on the wall surface of canyon interior.Only one large clockwise vortex exists in the lower frontage and higher back canyon.The velocity of inner airflow decreases with the increase of canyon depth.The stagnation point will appear after the depth reaches a certain value.There will be a clockwise vortex in the interior of symmetric street canyon with the smaller intensity.The interior airflow field is transferred duality structure of one clockwise vortex and one counterclockwise vortex with the increase of canyon depth.When the canyon is very deep,the obvious stagnation point will appear in the symmetric street canyon.The airflow field of lower frontage and higher back canyon is advantageous to the translocation and diffusion of pollutants.Thus,this kind of building distributions should be designed in urban planning according to the leading wind direction.